1. Field of the Invention
The present invention relates generally to an arrangement for converting divergent polarized laser emissions into an at least less divergent beam for feedback-free coupling into, for example, an optic fiber.
2. Description of the Related Art
An arrangement for converting a divergent semiconductor laser emission into a convergent beam for coupling with a monomode optic fiber is known from the publication IEEE Journal of Lightwave Technology, Volume LT-1, No. 1, pages 121-130, March 1983. In the disclosed arrangement, a first lens is composed of a spherical lens element of yttrium-iron-garnet (YIG). A Faraday rotator of the disclosed arrangement is composed of the first spherical lens element with a ring magnet surrounding the lens. A second lens is composed of a rod-shaped, graded index lens having end faces disposed perpendicularly to the propagation direction of the laser emission. A polarizer is arranged in the propagation direction of the laser emission following the second lens in the convergent region of the laser emission. The laser emission, accordingly, is capable of being coupled into a monomode fiber.
Other, known feedback-free arrangements are disclosed in (IEEE Journal of Lightwave Technology, Volume LT-4, No. 2, February 1986; Conference on Optic Fiber Communications (OFC) 1985, San Diego, Calif., paper PD 13-1, 1985; and Conference on Optic Fiber Communication (OFC) 1986, Atlanta, ME 4 1986. In contrast to these other known arrangements, wherein the Faraday rotator is arranged between the spherical first lens and a second lens in the form of a graded index lens or spherical lens and is composed of a disk of YIG inside a ring magnet and arranged obliquely relative to the propagation direction of the laser emission, the first-described arrangement has the advantage that one element is saved.
Such feedback-free arrangements are required for optical communications technology in which semiconductor lasers are becoming increasingly more important. Transmission rates into the gigabit region are possible as a result of the excellent modulation properties of semiconductor lasers. Such high performance transmission systems require high demands on the stability of the operating properties of the semiconductor laser.
Radiation beamed back or reflected back, in the direction of the semiconductor laser, for example laser emissions reflected within or outside of the arrangement, disturbs the operating properties of the laser in an undesirable way when the reflected emission is not blocked but is coupled back into the laser. In particular, reflected radiation leads to fluctuations in the spectrum, the line width, and the instrinsic noise of the laser (see IEEE Journal Quantum Electronics, Volume QE-16, pages 347-355, 1980; IEEE Journal Quantum Electronics, Volumes QE-18, pages 543-555, 1982; and Appl. Phys. Lett., Volume 445, No. 6, pages 597-599, 1984).
The feedback of the reflected radiation on the semiconductor laser can be largly feedback-free arrangements set forth above.